1. Field of the Invention
The present invention relates to a method for producing a precursor pitch for carbon fiber in which coal tar soft or middle pitch having a softening point of 50.degree.-70.degree. C. is subjected to two stages of heat-treatments to form mesophase pitch.
2. Description of the Prior Art
The production of carbon fibers is generally classified into a method using synthetic fibers such as polyacryl nitrile, etc. as the raw material and a method using a petroleum pitch or a coal tar pitch as the raw material in view of the raw material. The former method has the drawbacks that the raw material fiber is expensive and the carbonization yield of the raw material fiber is low. In the latter method, there is the following drawback that the pitch having a good spinnability is poor in the infusibility but the pitch having a good infusibility is poor in the spinnability. Presently commercially available carbon fibers are almost formed by using petroleum pitch as the raw material. But, when the petroleum pitch is used as the raw material, it is essential to effect the removal of insoluble solid components for adjusting the raw material pitch and a variety of specific physical chemical treatments such as hydrogenation, heat treatment and the like and this pitch adjustment and the specific treatments need much labor and time.
When the coal tar pitch is used as the raw material, it is necessary to separate and remove microparticle free carbon having a diameter of less than 1 .mu.m contained in the pitch as the insoluble solid component and in order to improve the spinnability and infusibility, the specific adjustment of pitch, such as use of a plurality of solvents, hydrogenation, heat treatment and the like should be performed. In general, the following properties are demanded as the precursor for carbon fiber.
(1) Containing no insoluble solid components, such as ash, free carbon, etc. PA0 (2) Excellent heat stability. PA0 (3) High melt spinnability. PA0 (4) Infusible treatment is easy. PA0 (5) Carbonization yield is high. PA0 (6) Carbon fibers obtained by melt-spinning of the precursor pitch, subjecting the spun fiber to infusing and carbonizing treatments are excellent in the graphitization and orientation and have the enough strength and Young's modulus to be required in carbon fiber.